Significant normative and editorial updates (w3c#16)

* Significant normative and editorial updates
* XML Serialization algorithm revisions to improve real-world interop
  (all changes based on testing the prior algorithm using the
   https://github.com/travisleithead/xmlserialization-polyfill)
* Editorial updates to provide a more friendly introduction
* various cleanup of "domintro" sections with expanded clarifying text

* Fix small merge issues; added to the revision history

Author: travisleithead